During the useful life of a gas turbine, maintenance operations are necessary for guaranteeing the correct functioning of the turbine itself. During these periodical operations, controls and inspections are done and damaged or worn parts or components are substituted.
The parts of a turbine which are most subject to wear, are the turbine blades as they undergo mechanical stress at a high temperature and are also subject to hot corrosion due to the hot gases with which the turbine operates. Therefore, there exists the necessity of periodical inspections of the turbine blades to control their integrity and functionality. During programmed maintenance operations, in order to be able to inspect the blades of the turbine, it is usually necessary to rotate the blades of the turbine, which is done by rotating the whole turbine rotor. This is applied especially in the case of a boroscopic inspection where the turbine is decoupled from the generator, so that the blades can be inspected by means of a boroscope.
Large turbo machinery rotors, particularly of large gas turbines, need to be rotated at a very low rotational speed and to an exact position during boroscopic inspection in order to precisely carry out operations on the rotor, such as mechanical rotor maintenance, rotor balancing or rotor alignment. Typically, boroscopes are used for this kind of inspection work, where the area to be inspected is inaccessible by other means: boroscopes are optical devices comprising illuminating means for the illumination of the remote object to be inspected, such that an internal image of the illuminated object is obtained and is further magnified to be presented to the viewer's eyes.
Boroscopes are commonly used in the visual inspection of industrial gas turbines, as gas turbines require particular attention because of safety and maintenance requirements. Boroscope inspection can be used to prevent unnecessary maintenance, which can become extremely costly for large gas turbines.
Because of the reduced visibility, it is necessary to rotate the rotor (shaft) of the turbine to be able to inspect all of its blades. Typically, the rotor is manually actuated, as it is not accessible the shaft of the low pressure turbine is then rotated manually by acting on the portion of the turbine shaft which has been decoupled at the loading joint.
Different boroscope devices used for the inspection of turbomachines are known in the state of the art. For example, document EP 2495553 A2 discloses a portable boroscope assembly used for the inspection of turbomachine blades. Also known in the art is document US 2012/0204395 A1, disclosing a method for inspecting and/or repairing a component in a gas turbine engine, by using a boroscope. Also, document US 2012/0285226 A1 discloses a system having a wear-indicating mark applied to a portion of surface of an internal component in a turbine, this mark being visually discernible through boroscopic inspection. Also known in the art, as per document EP 1749979 A2, is a system comprising a crank rotation mechanism having a reducer group for rotating, in particular manually, the shaft of the turbine to allow the inspection of blades by means of a boroscope. However, all these documents of the prior art that have been cited move the rotor (shaft) of the turbine manually, therefore being not accurate and being costly and time consuming.
Another system for rotating a shaft of a turbine, known in the art, is for example the one shown in document U.S. Pat. No. 4,193,739, where a device for turning a rotor of a gas turbine engine is disclosed for inspection purposes, comprising a nozzle that directs a jet of air onto the blades to turn the rotor. Also, the device comprises a rod that can move axially and that can stop the rotor. However, this system is not accurate and also requires human exertion, which makes it costly and time consuming. Also, this system is not able to provide a variable speed control on the rotor speed, in order to accurately effect boroscopic inspections in the gas turbine.
Also known in the art is document US 2010/0280733 A1, showing a gas turbine whose rotor speed is controlled by means of a controller, so that the shutdown of the rotor is controlled by controlling the rotor speed. Again, this kind of system cannot be properly used for accurate boroscopic inspection, where an accurate and specific positioning of the rotor is required. Moreover, boroscopic inspection requires variable speed (higher speed first and then, when a more accurate approach is done, a lower speed), which cannot be provided by this system.
For moving the rotor of a gas turbine it is also known to use a hydraulic device, typically a hydraulic cylinder, comprising a piston moving within the cylinder by the actuation of oil, typically. The unidirectional force obtained from this device actuates a rotor barring wheel having both a linear and radial movement. The problem of such a device is that, as it is actuated by oil, it is hard to control its movement. Also, oil is not the preferred actuating medium to use, as cleaning has to be done on a regular basis, which therefore requires time and extra cost.
Therefore, it is advantageous to provide a system for a gas turbine that is able to actuate the rotor of the gas turbine, such that the rotor can be remotely and automatically turned in variable speed and stopped at a specific and accurate position.
The present invention is directed towards providing these needs.